Evaluate Radar Data Assimilation in Two Momentum Control Variables and the Effect on the Forecast of Southwest China Vortex Precipitation

Based on the Weather Research and Forecasting (WRF) Model and the three-dimensional variational (3DVAR) data assimilation system, this study investigates the effects of assimilation of radar reflectivity and radial velocity under different momentum control variables on the forecast of Southwest China Vortex precipitation. It is shown that the U-V control variable strengthens the wind speed and vorticity to be better matching the observation, while using psi-chi as the control variable will produce too large increments which are unphysical. The root mean square errors (RMSE) of radar radial velocity are around 2.4 m/s in the experiment using psi-chi control variables, while the RMSE are below 2 m/s in the experiment with U-V control variables. The composite reflectivity from the analysis of the U-V control variables matches better with the observation than that from the analysis of the psi-chi control variables, i.e., the forecast rain band location under U-V control variables is more accurate. psi-chi control variable enhances the cold high-pressure system in near surface, while the U-V control variable is not significant. The water vapor flux convergence in the lower layers of the psi-chi control variable is overestimated leading excessive precipitation in the forecast. The Equitable Threat Score (ETS) of the U-V control variable is about 0.1 higher than psi-chi control variable. In summary, the U-V control variable is superior to the psi-chi control variable in terms of analysis and forecasting about Southwest China Vortex precipitation.